The invention relates to wear resistant articles, and in particular brake discs, e.g. for use on aircraft. Additionally, the invention relates to an improved method for the manufacture of such articles and in particular to improving the wear resistance of carbon-carbon composite disc shaped bodies.
An aircraft brake assembly is a pack of annular stator discs interleaved with annular rotary rotor discs together with thruster means to bring the stator and rotor discs into frictional engagement. There is a need for discs having faces of improved wear resistance.
Mechanisms for the wear of carbon have been proposed by Aswasthi and Wood (Advanced Ceramic Materials, volume 3, no. 5, 1988, pp449-451), Murdie et al (Carbon, 29, (1991) 335-342), and Hutton et al (Carbon, 37, (1999) 907-916). High wear rates at carbon-carbon friction surfaces are generally associated with the generation and loss of abrasive wear particles from the surfaces at low brake energy conditions. Low wear in carbon-carbon is generally associated with the retention of wear particles on the surface and which are modified through a shearing action to form a film at the friction surface under high energy brake conditions.
Ronyak and Leffel, in their European Patent Publication No. 0721835 A2, disclose a shaped filamentary structure and method of making same, in which segments of a disc, which is to form a brake disc, comprise multi-directional filaments extending in at least two directions. The filaments are arranged such that they extend generally parallel to the bearing or wear surface of the disc. The segments are assembled such that they define an overlapping contiguous structure of helical form, the whole being needlepunched to cross-link the layers and provide a preform suitable for a densification process.
Lawton and Smith, in their European Patent Publication No. 0424988 A1, disclose a method of producing a shaped filamentary article which may be used in an aircraft braking system. In order to produce such a structure, segments of an annulus are laid on to a substrate to form a layer, each segment comprising either radial or tangential fibres, and subsequently the segments are needlepunched to the substrate whilst effecting relative rotation of the substrate and layers relative to the needlepunch head. In such a fashion a disc is built up which, once the desired number of layers have been built up, may be peeled from the substrate to allow for further treatment such as densification.
It is one object of this invention to provide a body, such as a brake disc, having the fibres oriented in such a way that the disc has improved wear resistance to give increased brake life or reduced weight for the same life and other advantages.
According to the invention in one aspect there is provided an annular body comprising at least one wear surface portion, the portion comprising a plurality of layers held in a matrix, each layer consisting essentially of fibres which extend generally in one direction and parallel to each other and from the centre of the body to the periphery, whereby the wear of that surface portion is improved.
Typically in a disc having a pair of opposed faces, both of which may have the same arrangement and, in an embodiment of the invention, an intermediate portion of the disc may comprise a substantially identical or a distinct arrangement to that of the wear surface portion(s).
Each layer may comprise a plurality of segmental portions, each having an inner periphery and an outer periphery, the parallel fibres preferably extending therebetween.
Typically the segmental portions will have an annular size that does not give a whole number of segments in one layer to avoid alignment of segment joints across adjacent layers, resulting in a spiral lay up through the thickness of the disc. The segment angle may range widely from say 20xc2x0 to 70xc2x0, more preferably about 40xc2x0 to about 50xc2x0, most preferably 48xc2x0.
Preferably the generally parallel long fibres are connected by relatively short connecting lengths of fibre, which may be called the non-portion-spanning fibres. Methods of introducing the short connecting fibres include needle punching the long fibres using barbed needles, or more preferably, needle punching a staple felt of short crimped fibres to the long fibres, the short fibres in the staple felt being aligned generally perpendicular to the long fibres.
The fibres may be made from a stabilised or oxidised form of rayon, polyacrylonitrile; wool; pitch, or the like; or any combination that will, on heat treatment, produce carbon fibres. The long fibres in the preform may range in length according to the size of the body to be formed. The length of the connecting short fibres will be in the usual range. There are several well known methods for the manufacture of composite friction materials, especially those containing carbon fibres in a carbon matrix. Commercially available composites comprise random chopped fibre, 2D, 3D or needled fibre architectures in a matrix derived from pitch or resin precursors, or by chemical vapour infiltration or deposition or any combination of the above.
In another aspect therefore the invention provides a method of forming an annular body comprising a plurality of layers held in a carbon matrix, the method comprising:
forming a sheet consisting essentially of longitudinally extending generally parallel long fibres;
carbonising the fibres in the sheet to form a carbonised sheet;
cutting from the carbonised sheet segmental portions having an outer curved edge and an inner curved edge, and in which the fibres lie generally parallel to each other and extend from the inner edge to the outer edge;
arranging the segmental portions in a circular array to form an annular layer;
building up a plurality of the layers to form a preform; and
densifying the preform by introducing carbon containing vapour to form the carbon matrix.
The annular body may comprise an aircraft brake disc or a brake or clutch disc.
A further aspect of the invention resides in a method of forming an aircraft brake disc comprising:
forming a sheet consisting of longitudinally extending generally parallel long fibres and relatively short lengths of fibre, the parallel fibres being connected by means of said short lengths of fibre;
carbonising fibres in the sheet to form a carbonised sheet;
cutting from the carbonised sheet segmental portions having an outer curved edge and an inner curved edge, and in which the long fibres lie generally parallel to each other and extend from the inner edge to the outer edge;
arranging the segmental portions in successive abutting relation such that the portions describe a helical structure;
compressing axially the so-formed helical structure; and
densifying the compressed helical structure by introducing carbon containing vapour to form a carbon matrix.
While we do not wish the invention to be limited in any way by the following theory, our evaluations suggest that any one or more of the following three factors may cause or contribute to the beneficial result. First, the fact that the fibres extend generally parallel to each other and (nominally) radially across the wear face; second the fact that such fibres are held together by connecting cross-short lengths formed by teased out portions of the fibres which extend perpendicular to the longitudinal radial fibres; and third, the orientation and interconnection of fibres gives a structure with accessible open porosity that traps wear debris to assist the development and retention of a surface debris film.